Tools › Exposure & light

Flash guide number

A guide number (GN) ties distance and aperture together: GN = distance × f-number. Enter two and get the third, for any ISO and power setting.

How it works

Guide numbers are given in metres at ISO 100. Doubling the ISO multiplies the GN by 1.41; half power divides it by 1.41.

Bounce flash loses one to three stops on the way to the ceiling and back: open up or raise the ISO.

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Change one setting and see what the others must do: equivalent exposures and the EV.

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